The present invention relates to optical disks. More particularly, the present invention relates to optical disks on which data relating to playback power has been recorded.
Optical disks on which it is possible to record data at a high density, and from which it is possible to playback data at a high speed, are of increasing interest as data storage devices for computers. Optical disks having a diameter of 5.25 or 3.5 inches, i.e. conventional magneto-optical or phase change disks, are capable of rewriting recorded data and are internationally standardized as ISO norms. Recently, norms for digital video disks ("DVD") have also been standardized, and further acceleration is expected in the application of optical disks to the multimedia field.
A concave or convex groove is conventionally formed as a spiral in an optical disk. This groove guides a laser beam which is projected from an optical pickup. This is commonly referred to as "tracking," and the concave or convex groove is referred to as a "guide groove." Through use of this guide groove, data is systematically recorded and arrayed in a row. Moreover, by faithfully tracking a recorded row of data, the data is accurately played back.
In the ISO norm for guide grooves, a concave portion viewed from an optical pickup side is referred to as a "land." On the other hand, a convex portion as seen from the optical pickup side is referred to as a "groove." Data is recorded on one of either lands or grooves. The distance from the center of a land (or groove), to the center of an adjacent land (or groove) is referred to as "track pitch."
The width W of a groove, putting the width of the top of the groove as Wtop, and the width of the bottom of the groove as Wbottom, is defined by W=(Wtop+Wbottom)/2. Moreover, the height from groove bottom to groove top, namely, a difference in level of a land portion and a groove portion, is called "groove depth." The dimensions of groove width, of the type in which recording is performed in land portions, is on the order of 0.3-0.6 .mu.m. Moreover, if the wavelength of a beam used for record playback is .lambda., and the refractive index of a substrate is n, the groove depth is on the order of EQU .lambda./(10.multidot.n) to .lambda./(6.multidot.n).
In an optical disk, preformatted signals of track numbers or sector numbers other than the guide groove are formed in advance as rows of marks which become convex as seen from the optical pickup side, in other words, as rows of pits.
In recent years, opportunities have increased for storage of large amounts of data, primarily for pictures and the like. Such circumstances for recording at higher densities and playing back with high accuracy have strict requirements, and various approaches to address these requirements are being attempted.
During recording, if only a high energy region near the center of the beam is used, suitably small marks can, in principle, be formed. For example, in a playback-only (ROM) type of optical disk, concave-convex pits are formed as follows. First, a photoresist surface is applied to a glass plate and irradiated with a narrow laser beam modulated with data to be recorded. Photo-reactions are then caused to take place locally in the photoresist. After a development process, a concave-convex pattern is formed on the photoresist surface. This pattern is then nickel plated.
After separation of the nickel plating from the concave-convex pattern, a nickel plate is obtained with concave-convex pits formed in the surface. This nickel plate is then set in a metal mold for injection molding, and an optical disk substrate having concave-convex pits is formed by molding. A reflective layer is then formed on the optical disk substrate thereby completing the optical disk. In the phase change type or magneto-optical type of optical disk, small marks can be formed during recording if only the high temperature region close to the beam center is arranged to form the recording marks. However, problems in the above process develop with regard to playback. In order to optically detect the concave-convex pits or magneto-optical marks in a playback beam spot, it is not possible to playback only data which is in a portion of the beam. Accordingly, a first problem of high density recording playback is to accurately playback data having continuous small marks.
In order to solve this problem, the following approach is considered. First, track pitch is made closer. Standard track pitch is 1.6 .mu.m; however recent trials have been carried out to make the track pitch narrower, i.e. 1.4 .mu.m or 1.2 .mu.m. Furthermore track pitch of 1.0 .mu.m has been reported. However, in the case of an optical pickup loaded with an objective lens having a numerical aperture (NA) of about 0.5-0.6, when the track pitch is made narrower than about 1.4 .mu.m, data which is recorded on adjacent tracks is simultaneously read out. This is termed "optical crosstalk." Because of this, the recordings cannot be accurately played back.
To afford accurate playback, the wavelength of the playback beam is shortened. In other words, the extent to which the beam is shortened is proportional to the wavelength of the beam. Consequently, playback beam spot size is reduced and recorded data is played back at high density. Thus, the problem of optical crosstalk is avoided.
However, the wavelengths of semiconductor lasers which are presently used as light sources for optical pickups are limited from the standpoint of stability in output power. For example in the prior art, semiconductor lasers were generally of a wavelength of 830 nm. More recently, 680 nm has become the mainstream. That is, by shortening the wavelength to 680 nm, the amount of reduction in beam size is about 18%. However, it is difficult to reduce the beam spot size in rapid progression to 1/2 or 1/3 of its present form.
A record playback method and medium have been invented by which it is possible to accurately playback data recorded at a suitable high density, even with a playback beam of a conventional size. The basic concept of this invention is generally as follows.
The temperature of the recording medium rises due to playback beam irradiation. Because the medium is moving, the temperature of an advancing direction side of the irradiated playback beam spot becomes a higher temperature. By using this characteristic of temperature distribution and by masking a portion within the spot so that it is not seen by the optical pickup, only a portion of the optical medium which can be seen is played back. Thus, playback of only the data of a small portion of the interior of the spot is possible. That is, the playback beam spot size becomes substantially reduced and data recorded at a high density can be accurately played back.
More specifically, a smaller aperture provides: (1) a low temperature portion within a playback beam spot which becomes an aperture (high temperature portion becomes masked), (2) a high temperature portion within the playback beam spot which becomes an aperture (low temperature portion becomes masked), or (3) a high temperature portion which becomes an aperture (but within the aperture portion, a highest temperature portion is masked). Mask formation is also accomplished by a change in transparency through phase change in which a change of magnetization direction results from a change in magnetic coupling force.
The principles of a conventional device in which a change in magnetization direction is controlled through a change in magnetic coupling force is described with reference to FIGS. 4 (PRIOR ART) and 5 (PRIOR ART). FIG. 5 is a cross section of a principal portion of a conventional magneto-optical disk 20 and also of a temperature distribution of a portion which is irradiated by a playback beam 51. Magneto-optical disk 20 has two magnetic layers, namely a mask layer 55 and a recording layer 56. Data is recorded in a perpendicular direction of magnetization 57 in recording layer 56. Mask layer 55 is disposed above recording layer 56.
As illustrated in FIG. 5, playback beam 51 irradiates the surface of magneto-optical disk 20 and forms a temperature distribution 52. Reference numerals 53 and 54 denote, respectively, high and low temperature regions with respect to a predetermined value.
The direction of magnetization for mask layer 55 is in the plane of mask layer 55 for regions having a temperature lower than a predetermined temperature, i.e. low temperature regions. However, the direction of magnetization for mask layer 55 is oriented in the same direction as the direction of magnetization of recording layer 56 through an exchange of coupling force with recording layer 56, for regions above a predetermined temperature, i.e. high temperature regions. The predetermined temperature arises in small regions close to a center of temperature distribution 52 through irradiation by playback beam 51.
Turning now to FIG. 4, conventional magneto-optical disk 20 moves in disk movement direction 44 to advance a plurality of marks beneath playback beam spot 41. During operation, playback beam spot 41 irradiates a desired mark 46 to be read out and a masked mark 45. The desired mark 46 is within high temperature region 43 while masked mark 45 is within low temperature region 42. Low temperature region 42 is a region where the direction of magnetization is in the planar direction of the mask layer 55. As a result, only desired mark 46 is read out. Thus, even though a plurality of marks may be within the playback beam spot, marks outside the high temperature region are not read out.
Methods of recording and playback of an optical disk include a constant angular velocity (CAV) method and a constant linear velocity (CLV) method. In the CAV method an optical disk is rotated at a constant rpm and thus, the CAV method is suitable for recording and playback at high speeds. In this method, linear speeds differ at an inner circumference and an outer circumference of the disk. During recording, recording beam power is changed according to a radial position on the disk such that a temperature rise in the medium due to irradiation by the beam is maintained constant regardless of radial position. However, the intensity of the playback beam is constant regardless of radial position because the playback signal is at a level which is greater to a degree.
From the above description of recording beam intensity, in a case where playback beam intensity is maintained constant, a temperature increase of the medium differs at the inner circumference and the outer circumference. In a super resolution medium, because the size of the aperture is set according to temperature of the medium, during playback at a constant playback beam intensity, the aperture at the inner circumference becomes large because a temperature rise at the inner circumference becomes large. Moreover, the aperture becomes small at the outer circumference because the temperature rise is small.
For example, a conventional playback state of an inner circumferential part of an optical disk is illustrated in FIG. 3 (PRIOR ART). As illustrated, playback beam spot 31 irradiates partial mark 34 and desired mark 35. In addition, a region of perpendicular magnetization 33 covers desired mark 35 and a portion of partial mark 34, while a region of planar magnetization 32 partially covers partial mark 34. Thus, partial mark 34 and desired mark 35 are simultaneously read out, and a source of error is presented. In other words, optimum playback conditions are not realized over the whole surface. Consequently, playback beam intensity must change according to radial position.
However, thermal capacity varies according to a particular construction of an optical disk, even for optical disks of similar type, and thermal capacity differs from manufacturer to manufacturer. Moreover, even if construction of optical disks is the same, recording layer thickness differs between individual disks, and therefore, scatter exists in their respective thermal capacities.
In view of the above, optimum beam intensity is considered to change for each manufacturer. Moreover, even from the same manufacturer, individual disks may differ. In particular, for a magnetically-induced super resolution medium which uses a magnetic layer as a recording layer, because optimum playback beam intensity differs according to the thickness of the magnetic layer and also construction, there is a tendency for scatter of optimum playback beam intensities to become even larger between disks. Accordingly, for a light beam to perform magnetically-induced super resolution playback, a problem is that optimum playback beam intensity for each individual disk is not set and accurate playback is not possible.
Furthermore, it is difficult to make thickness and composition of recording layers completely uniform with respect to radial direction in a single disk. Some degree of variation, i.e. bias, unavoidably exists. This bias does not differ uniformly for individual disks. In particular, in the case of optical disks for performance of the above-mentioned super resolution playback, even if two optical disks have similar optimum playback beam intensities in some radial positions, mutual differences in optimum beam intensity are obtained when radial positions change. In such cases, a problem is that in each disk, optimum beam intensity cannot be set according to radial position and accurate playback is not possible.